Bowed crests for milled tooth bits

ABSTRACT

A drill bit comprising a bit body, at least one roller cone rotatably mounted on the bit body. The cone has a plurality of milled teeth at selected locations on the cone. At least one of the milled teeth comprises a substrate having a convex crest and a layer of hardfacing applied to said convex crest. The convex crest is adapted to produce at least one of a convex axial stress distribution, a substantially even axial stress distribution, and a substantially smooth axial stress distribution.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] Not applicable.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0002] Not applicable.

BACKGROUND OF THE INVENTION

[0003] 1. Field of the Invention

[0004] The invention relates generally to earth-boring bits used todrill a borehole for the ultimate recovery of oil, gas or minerals. Moreparticularly, the invention relates to roller cone rock bits and to animproved cutting structure for such bits. Still more particularly, theinvention relates to a cutter element having a bowed crest geometrywhich provides for a more uniform stress distribution.

[0005] 2. Background Art

[0006] The success of rotary drilling enabled the discovery of deep oiland gas reserves. The roller cone rock bit was an important inventionthat made that success possible. The original roller-cone rock bit,invented by Howard R. Hughes, U.S. Pat. No. 930,759, was able to drillthe hard caprock at the Spindletop field, near Beaumont, Tex.

[0007] That invention, within the first decade of the twentieth century,could drill a scant fraction of the depth and speed of modem rotary rockbits. If the original Hughes bit drilled for hours, the modern bitdrills for days. Bits today often drill for miles. Many individualimprovements have contributed to the impressive overall improvement inthe performance of rock bits.

[0008] Roller-cone rock bits typically are secured to a drill string,which is rotated from the surface. Drilling fluid or mud is pumped downthe hollow drill string and out of the bit. The drilling mud cools andlubricates the bit as it rotates and carries cuttings generated by thebit to the surface.

[0009] Roller-cone rock bits generally have at least one, and typicallythree roller cones rotatably mounted to a bearing on the bit body. Theroller cones have cutters or cutting elements on them to induce highcontact stresses in the formation being drilled as the cutters roll overthe bottom of the borehole during drilling operation. These stressescause the rock to fail, resulting in disintegration and penetration ofthe formation material being drilled.

[0010] Operating in the harsh down hole environment, the components ofroller-cone rock bits are subjected to many forms of wear. Among themost common forms of wear is abrasive wear caused by contact withabrasive rock formation materials. Moreover, the drilling mud, ladenwith rock chips or cuttings, is a very effective abrasive slurry.

[0011] Many wear-resistant treatments are applied to the variouscomponents of the roller-cone rock bit. Among the most prevalent is theapplication of a welded-on wear-resistant material or “hardfacing.” Thismaterial can be applied to many surfaces of the rock bit, including thecutting elements.

[0012] U.S. Pat. No. 4,262,761 discloses a milled steel tooth rotaryrock bit wherein one or more holes are drilled into the crest of thetooth-shaped cutting structure. Tungsten carbide rods are positioned inthe holes and hardfacing is applied to the tooth. The hardfacing isapplied across the top of the tooth crest and acts to hold the tungstencarbide rods in place. The rods are inserted in holes parallel and closeto one flank of the tooth so that the entire length of the carbide rodscan be attached to the hardfacing by burning the hardfacing through tothe carbide rods. Wear on the tooth will proceed along the side of thetooth not reinforced with the carbide rods and a self-sharpening effectis enhanced by the strength of the carbide rods. The carbide rods andholes therefore can be relatively inexpensive, since close tolerancefinishing is not required.

[0013] U.S. Pat. No. 5,152,194 discloses a milled tooth roller cone rockbit consisting of chisel crested milled teeth with generously radiusedcorners at the ends of the crest. A concave depression is formed in thecrest between the radiused ends. A layer of hardfacing material formedover each tooth is thicker at the corners and in the concave depressionsin the crest to provide a means to inhibit wear of the hardfacing as thebit works in a borehole.

[0014] U.S. Pat. No. 5,311,958 discloses an earth-boring bit that isprovided with three cutters, two of the three cutters are provided withheel disk cutting elements defined by a pair of generally oppositelyfacing disk surfaces that generally continuously converge to define acircumferential heel disk crest. One of the two cutters having heel diskelements is further provided with an inner disk A cutting element.

[0015] U.S. Pat. No. 5,492,186 discloses an earth boring bit rotatablecutter having a first hardfacing composition of carbide particlesselected from the class of cast and macrocrystalline tungsten carbidedispersed in a steel matrix deposited on the gage surface of at leastsome of the heel row teeth. A substantial portion of these particles arecharacterized by a high level of abrasion resistance and a lower levelof fracture resistance. A second hardfacing composition of carbideparticles selected from the class of spherical sintered and sphericalcast tungsten is dispersed in a steel matrix deposited over at least thecrest and an upper portion of the gage surface to cover the corner thattends to round during drilling. A substantial portion of the particlesof this composition are characterized by a high level of fractureresistance and a lower level of abrasion resistance.

[0016] U.S. Pat. No. 5,868,213 discloses a steel tooth, particularlysuited for use in a rolling cone bit, includes a root region, a cuttingtip spaced from the root region and a gage facing surface therebetween.The gage facing surface includes a knee, and is configured such that thecutting tip is maintained at a position off the gage curve. Sopositioned, the cutting tip is freed from having to perform anysubstantial cutting duty in the comer on the borehole comer, and insteadmay be configured and optimized for bottom hole cutting duty. The kneeon the gage facing surface is configured and positioned so as to serveprimarily to cut the borehole wall. It is preferred that the knee bepositioned off gage, but that it be closer to the gage curve than thecutting tip.

[0017] U.S. Pat. No. 6,206,115 discloses an earth-boring bit having abit body with at least one earth disintegrating cutter mounted on it.The cutter is generally conically shaped and rotatably secured to thebody. The cutter has a plurality of teeth formed on it. The teeth haveunderlying stubs of steel which are integrally formed with and protrudefrom the cutter. The stubs have flanks which incline toward each otherand terminate in a top. A carburized layer is formed on the flanks andthe top to a selected depth. The stub has a width across its top fromone flank to the other that is less than twice the depth of thecarburized layer. A layer of hardfacing is coated on the tops and flanksof the stub, forming an apex for the tooth.

[0018] U.S. Pat. No. 6,241,034 discloses a cutter element for a drillbit. The cutter element has a base portion and an extending portion andthe extending portion has either a zero draft or a negative draft withrespect to the base portion. The non-positive draft allows more of theborehole bottom to be scraped using fewer cutter elements. The cutterelements having non-positive draft can be either tungsten carbideinserts or steel teeth.

[0019] Referring now to FIG. 1, which illustrates a milled tooth rollercone rock bit generally designated as 10. The bit 10 consists of bitbody 12 threaded at pin end 14 and cutting end generally designated as16. Each leg 13 supports a rotary cone 18 rotatively retained on ajournal, optionally cantilevered from each of the legs (not shown). Themilled teeth generally designated as 20 extending from each of the cones18 may be milled from steel. Each of the chisel crested teeth 20 forms acrest 24, a base 22, two flanks 27, and tooth ends 29.

[0020] Hardfacing material may be applied to at least one or each of theteeth 20. In one embodiment, the application of hardfacing is appliedonly to the cutting side of the tooth as opposed to the other flanks 27and ends 29 of the teeth 20. In another embodiment, the hardfacing maybe applied to all the flanks 27 and ends 29 of the teeth 20.

[0021] The rock bit 10 may further include a fluid passage through pin14 that communicates with a plenum chamber (not shown). In oneembodiment, there are one or more nozzles 15 that are secured withinbody 12. The nozzles direct fluid from plenum chamber (not shown)towards a borehole bottom. In another embodiment, the rock bit 10 has nonozzles 15. In another embodiment, the upper portion of each of the legsmay have a lubricant reservoir 19 to supply a lubricant to each of therotary cones 18 through a lubrication channel (not shown).

[0022] Turning now to the prior art of FIGS. 2A and 2B, conventionalhardfaced chisel crested teeth generally designated as 40, when theyoperate in a borehole for a period of time, wear on the corners 44 ofthe teeth. The prior art tooth consists of a crown or crest 41 havinghardfacing material 42 across the crest and down the flanks 43terminating near the base 45 of the tooth 40.

[0023]FIG. 2C shows the prior art tooth of FIG. 2A with a typical axialstress distribution. The prior art teeth (40) typically have a concaveaxial stress distribution (50) as shown in FIG. 2C.

[0024] As heretofore stated the hardfacing material 42 transitioningfrom the crest 41 towards to the flanks 43 may be very thin at thecomers of the conventional teeth 40. Consequently, as the tooth wears,the hardfacing, since it may be very thin, may wear out quickly, andthus expose the underlying steel 47 of the tooth 40. Consequently,erosion voids (not shown) could invade the base metal 47 since it isusually softer than hardfacing material 42.

SUMMARY OF THE INVENTION

[0025] One aspect of the invention is a drill bit comprising a bit body,at least one roller cone rotatably mounted on the bit body. The cone hasa plurality of milled teeth at selected locations on the cone. At leastone of the milled teeth comprises a substrate having a convex crest anda layer of hardfacing applied to said convex crest. The convex crest isadapted to produce at least one of a convex axial stress distribution, asubstantially even axial stress distribution, and a substantially smoothaxial stress distribution.

BRIEF DESCRIPTION OF THE DRAWINGS

[0026]FIG. 1 is a perspective view of a milled tooth rotary cone rockbit with hardfacing material on each tooth;

[0027]FIG. 2A is a cross-sectional prior art view of a toothillustrating the crest and hardfacing of the tooth;

[0028]FIG. 2B is a cross-sectional prior art view of a worn toothillustrating destructive voids in the hardfacing and base metal materialat the corners of the crest of the tooth;

[0029]FIG. 2C is a cross-sectional prior art view of a toothillustrating the axial stress distribution, crest, and hardfacing of thetooth;

[0030]FIG. 3 is a cross-sectional view of an improved hardfaced chiselcrested milled tooth;

[0031]FIG. 4 is a diagrammatic cross-section of a tooth of a 9⅞ inchmilled tooth rotary cone rock bit;

[0032]FIG. 5 is a cross-sectional view of another configuration of animproved hardfaced milled tooth;

[0033]FIG. 6 is a perspective view of a single chisel crested milledtooth with hardfacing in a thicker layer around rounded comers of thetooth adjacent the flank and end faces of the tooth;

[0034]FIG. 7 is a cross-sectional view of the axial stress distributionof an improved hardfaced chisel crested milled tooth; and

[0035]FIG. 8 is a cross-sectional view of the axial stress distributionof another configuration of an improved hardfaced milled tooth;

DETAILED DESCRIPTION

[0036] Turning now to one embodiment illustrated in FIG. 3, the chiseltooth generally designated as 20 consists of, for example, a steelfoundation 21, forming flanks 27, ends 29 and a crest 24. Betweenrounded comers 26 is a convex portion 25 on the crest 24 of the tooth.The convex portion 25 enables hardfacing material 32 to be thicker atthe comers 26 of the crest 24, therefore providing for more durablecutting comers 26. Each of the comers 26 has a sufficient radius so thatthe thickness of the hardfacing material is assured as it transitionsfrom the crest 24 towards the ends 29 and the flanks 27 of the tooth 20.The hardfacing material may terminate at the base 22 of each of theteeth 20. The base 22 provides a termination point for the hardfacingmaterial 32 as it is applied over the crest ends and flanks of each ofthe teeth 20.

[0037] By providing a convex portion 25 or rounded geometry and roundedcomers 26 at the end of the crested tooth, the hardfacing material maybe applied more generously at the comers 26 of the crest and at asufficient thickness in the center of the crest to produce a generallyflat crest 24. The geometry at the comers 26 assures a thick applicationof hardfacing material at a vulnerable area of the tooth.

[0038] One suitable hardfacing material and a method of its applicationis described in U.S. Pat. No. 4,836,307 to Keshavan et al and isincorporated herein by reference in its entirety.

[0039] Referring now to the cross-sectional example of FIG. 4, a typicaltooth 20 formed from a cone of a 9⅞ inch diameter milled tooth rollercone rock bit could, for example, have a tooth height “A” of about 0.5to about 1.5 inches, in one embodiment, 0.72 inches, and a width “B” ofabout 0.5 to about 1.0 inches, in one embodiment, 0.62 inches across thechisel crown of the tooth 20. The radius at the comers 26 may be betweenabout 0.02 and about 0.20 inches, in one embodiment, about 0.08 inches.The convex radius 25 may be between about 0.15 and 1.0 inches, in oneembodiment, 0.50 inches. The depth “C” of the convex radius may bebetween about 0.02 inches and about 0.20 inches, in one embodiment,about 0.05 inches.

[0040] In one embodiment, the crest 24 of the tooth 20 may besubstantially flat between radiused comers, the tooth having a variedhardfacing 32 thickness between radiused comers. In another embodiment,the crest 24 of the tooth 20 may be convex between radiused comers, thetooth having a constant hardfacing thickness between radiused comers. Inanother embodiment, the crest 24 of the tooth 20 may be convex betweenradiused comers, the tooth having a varied hardfacing 32 thicknessbetween radiused corners, wherein the hardfacing 32 is thicker at theradiused comers.

[0041] The hardfacing 32 may have a thickness along the ends 29, flanks27 and comers 26 between about 0.02 and about 0.18 inches, in oneembodiment a thickness of about 0.10 inches.

[0042] The thickness of the hardfacing at depth “D” and along the crest24 may be between about 0.04 and about 0.18 inches, in one embodiment adepth of about 0.10 inches (with respect to the example of FIG. 3).

[0043]FIG. 5 illustrates an alternative embodiment of the presentinvention wherein the chisel crest tooth generally designated as 120forms a crest 124 that transitions into ends 129 and flanks 127. Crest124 forms a convex shape 125, in one embodiment a bow, between corners126 that allows a substantially uniform thickness of hardfacing material132 across the crest 124. The hardfacing material 132 can also maintaina relatively thick layer across the corners 126 and down the ends 129and flanks 127 towards the cone 18 (shown in FIG. 1). One advantage maybe to maintain a uniform axial stress profile across the crest 124.Another advantage may be to provide a robust or thick hardfacingmaterial across the flanks 124 and ends 126 such that the tooth as itoperates in a borehole retains its integrity and sharpness as it worksin a borehole.

[0044] In another embodiment of the present invention (not shown), thechisel crest tooth, generally designated as 120 forms a crest 124 thattransitions into ends 129 and flanks 127. Crest 124 forms a convex shape125, in one embodiment a bow, between corners 126 that allows agradually decreasing thickness of hardfacing material 132 across thecrest 124, so that the thickness of the hardfacing material 132 isthickest across the comers and less thick in the middle between thecorners. The hardfacing material 132 can also maintain a relativelythick layer across the corners 126 and down the ends 129 and flanks 127towards the cone 18 (shown in FIG. 1). One advantage may be to maintaina uniform axial stress profile across the crest 124, or a convex stressprofile across the crest 124. Another advantage may be to provide arobust or thick hardfacing material across the flanks 124 and ends 126such that the tooth as it operates in a borehole retains its integrityand sharpness as it works in a borehole.

[0045] In another alternative embodiment, the flanks 127 and/or the ends129 may have a depression or concave portion (not shown) whereby thehardfacing material is thicker at the concave portion thus providing athicker area along the flanks 127 and/or the ends 129. In anotheralternative embodiment, the flanks 127 and/or the ends 129 may have aconvex portion (not shown) or a bow, whereby the hardfacing material iseither the same thickness or thinner at the convex portion (not shown).Hardfacing may terminate at base 122 at each of the mill teeth 120. Aconvex portion on the flanks 127 and/or the ends 129 may provideincreased tooth strength due to the larger amount of tooth substratematerial. A concave portion on the flanks 127 and/or the ends 129 mayprovide increased hardfacing thickness and increased tooth durabilitydue to the larger amount of tooth hardfacing material.

[0046] In another alternative embodiment, the tooth may have more thanone convex portions, or bows, along the crest, the corners may berounded in much the same manner as in FIGS. 3, 4, and 5 in order toassure a thickness at the corners of the tooth. In another alternativeembodiment, the flanks and/or the ends may have a concave portion, aconvex portion, or multiple concave and/or convex portions.Alternatively, the flanks and/or the ends may have a series ofdepressions to assure a robust layer of hardfacing along the ends andflanks. The hardfacing material may terminate on a groove or shoulder orrecess at the base of the tooth.

[0047]FIG. 6 illustrates a perspective view of one of the chisel crestedteeth 320 wherein the comers 330 of the tooth are rounded, so that aminimum thickness of hardfacing material 332 is on the comer 330, whichforms the junctions between the ends 329 and flanks 327. The steelfoundation (not shown) is covered by the hardfacing material 332. Thetop of the tooth 320 forms a crest 324. In one embodiment, the crest 324is convex, and in an alternative embodiment, the crest 324 issubstantially flat. The hardfacing material 332 terminates at the base322 of the tooth 320. The base 322 provides a termination point for thehardfacing material 332 as it is applied over the crest ends 329 andflanks 327 of each of the teeth 320. The hardfacing material 332 isapplied with a sufficient thickness over the entire tooth to improve itsintegrity and durability.

[0048] In an alternative embodiment, a milled tooth with a convex chiselcrest converging at both radiused ends could be hardfaced. In oneembodiment, the thickness of the hardfacing could remain substantiallyconstant across the crest as illustrated by the specific example of FIG.5. In another embodiment, the thickness of the hardfacing could varyacross the crest as illustrated by the specific example of FIG. 3.

[0049] In an alternative embodiment, a spherical or semi-sphericalsurface of a milled tooth could be hardfaced as long as the radiuses arewithin the general parameters set forth in FIG. 4, thereby assuring aminimum thickness of hardfacing and the enhanced durability of the toothas it works in a borehole.

[0050] In an embodiment such as shown in FIG. 6, each tooth 320, afterthe hardfacing 332 is applied, will appear outwardly with relativelystraight crest 324, ends 329, and flanks 327, the hardfacing having auniform termination point at the base 322 of the milled tooth 320. Inanother embodiment, one or more of the crest 324, ends 329, and flanks327 may have a rounded appearance.

[0051] In one embodiment of the invention, as shown in FIG. 1, the teeth20 have an axial crest 24. Axial crests 24 are so called because thecrest 24 generally is substantially aligned with the axis of rotation ofthe cone 18 that the tooth is located on. In an alternative embodiment,the teeth 20 may have a circumferential crest (not shown).Circumferential crests (not shown) are so called because the crest (notshown) generally is substantially oriented circumferentially about thecone 18 that the tooth is located on, or substantially aligned with acircumference of the cone 18 that the tooth is located on. Acircumferential crest (not shown) would have different loadingproperties and stress distribution than an axial crest 24 because acircumferential crest has a rolling action with the rock formationdownhole where only a portion of the crest interacts with the rockformation at one time, while for an axial crest 24, substantially theentire crest penetrates the rock formation at the same time. In anotherembodiment of the invention (not shown), the teeth 20 have a crest 24that is neither axial nor circumferential, but the crests 24 aresubstantially aligned with a line that is between the axis of rotationof the cone 18 that the tooth is located on and the circumference of thecone 18 that the tooth is located on. In another embodiment, the crests24 are substantially aligned with a line that is within about 40° (inany direction) of the axis of rotation of the cone 18 that the tooth islocated on. In another embodiment, the crests 24 are substantiallyaligned with a line that is within about 30° (in any direction) of theaxis of rotation of the cone 18 that the tooth is located on. In anotherembodiment, the crests 24 are substantially aligned with a line that iswithin about 15° (in any direction) of the axis of rotation of the cone18 that the tooth is located on.

[0052]FIG. 7 shows an embodiment of the tooth of FIG. 3 with an axialstress distribution. The tooth (20) may have a convex axial stressdistribution (52) as shown in FIG. 7. This convex axial stressdistribution (52) provides a higher level of axial stress in the middleof the crest (24) than at the comers (26) of the tooth (20). Advantagesof this convex axial stress distribution (52) may include aggressivepenetration of the rock formation while drilling.

[0053]FIG. 8 shows an embodiment of the tooth of FIG. 5 with an axialstress distribution. The tooth (120) may have a level axial stressdistribution (54) as shown in FIG. 8. This level axial stressdistribution (54) provides a substantially even level of axial stress inthe middle of the crest (124) as compared to the level of axial stressat the comers (126) of the tooth (120). Advantages of this level axialstress distribution (54) may include favorable tooth wear at the corners(126).

[0054] In one embodiment, shown in FIG. 7, the crest geometry is adaptedand/or designed to produce a convex axial stress distribution. Inanother embodiment, shown in FIG. 8, the crest geometry is adaptedand/or designed to produce a substantially even axial stressdistribution. In another embodiment, the crest geometry is adaptedand/or designed to gradually increase the thickness of the hardfacing onthe crest in relation to the magnitude of the axial stress. In anotherembodiment, the crest geometry is adapted and/or designed to produce asubstantially smooth axial stress distribution; some prior art crestgeometries could produce concave, or erratically shaped axial stressdistributions.

[0055] Other advantages of the invention may include one or more of thefollowing:

[0056] The larger radius at the comers of a crest of a milled toothenables a thicker layer of hardfacing at the comers of the crest of thetooth;

[0057] A thicker layer of hardfacing provided along a crest of a chiseltype milled tooth between radiused comers enhances the durability of thetooth as it operates in a borehole;

[0058] The radiusing of the comers adjacent the flanks and ends of thechisel crested teeth further strengthens the capability of the tooth toretain its hardfacing during downhole operations;

[0059] A convex substrate crest and a convex hardfacing crest provides auniform axial stress distribution across the crest;

[0060] A convex substrate crest and a flat hardfacing crest provides agradual increase in the hardfacing thickness, and thicker hardfacing atthe corners;

[0061] A convex substrate crest provides a convex axial stressdistribution;

[0062] A convex substrate crest provides a substantially even axialstress distribution;

[0063] A convex substrate crest provides a substantially smooth axialstress distribution;

[0064] A convex substrate crest provides a preferred loading condition;and

[0065] A convex substrate crest provides improved wear characteristics.

[0066] Other advantages of the invention will be apparent from theappended claims.

[0067] While the invention has been described with respect to a limitednumber of embodiments, those skilled in the art, having benefit of thisdisclosure, will appreciate that other embodiments can be devised whichdo not depart from the scope of the invention as disclosed herein.Accordingly, the scope of the invention should be limited only by theattached claims.

What is claimed is:
 1. A drill bit comprising: a bit body; at least oneroller cone rotatably mounted on said bit body; and a plurality ofmilled teeth at selected locations on the cone, wherein at least one ofsaid milled teeth comprises a substrate having a convex crest and alayer of hardfacing applied to said convex crest, wherein said convexcrest is adapted to produce at least one of a convex axial stressdistribution, a substantially even axial stress distribution, and asubstantially smooth axial stress distribution.
 2. The drill bit body ofclaim 1 wherein a crest of the layer of hardfacing is substantiallyflat.
 3. The drill bit body of claim 1 wherein a crest of the layer ofhardfacing is convex.
 4. The drill bit body of claim 3 wherein thethickness of the layer of hardfacing greater at least one corner than ina middle of the crest.
 5. The drill bit body of claim 4 wherein an axialstress distribution of the crest is convex.
 6. The drill bit body ofclaim 4 wherein an axial stress distribution of the crest issubstantially level.
 7. The drill bit body of claim 1 wherein an axialstress distribution of the crest is convex.
 8. The drill bit body ofclaim 1 wherein an axial stress distribution of the crest issubstantially level.
 9. The drill bit body of claim 1 wherein at leastone of said teeth has a flank, wherein said flank is convex.
 10. Thedrill bit body of claim 9 wherein at least one of said teeth has an end,wherein said end is convex.
 11. The drill bit body of claim 9 wherein atleast one of said teeth has an end, wherein said end is concave.
 12. Thedrill bit body of claim 1 wherein at least one of said teeth has aflank, wherein said flank is concave.
 13. The drill bit body of claim 12wherein at least one of said teeth has an end, wherein said end isconvex.
 14. The drill bit body of claim 12 wherein at least one of saidteeth has an end, wherein said end is concave.
 15. The drill bit body ofclaim 1 wherein at least one of said teeth has an end, wherein said endis convex.
 16. The drill bit body of claim 1 wherein at least one ofsaid teeth has an end, wherein said end is concave.
 17. The drill bitbody of claim 1 wherein said convex crest is substantially aligned withan axis of rotation of said roller cone.
 18. The drill bit body of claim1 wherein said convex crest is substantially aligned with a line that iswithin about 40° of an axis of rotation of said roller cone.
 19. Thedrill bit body of claim 1 wherein said convex crest is substantiallyaligned with a line that is within about 30° of an axis of rotation ofsaid roller cone.
 20. The drill bit body of claim 1 wherein said convexcrest is substantially aligned with a line that is within about 15° ofan axis of rotation of said roller cone.
 21. A method of forming milledteeth on a roller cone of a milled tooth roller cone rock bitcomprising: shaping a crest of at least one chisel shaped milled tooth,so that said crest comprises at least one convex profile from one cornerto an opposite corner of said crest, wherein said convex crest isadapted to produce at least one of a convex axial stress distribution, asubstantially even axial stress distribution, and a substantially smoothaxial stress distribution; and radiusing each of said corners at theends of the crest of said chisel shaped tooth.
 22. The method as setforth in claim 21 further comprising: applying hardfacing material oversaid at least one chisel shaped mill tooth, said hardfacing material isapplied over said radiused corners.
 23. The method as set forth in claim21 wherein there is a single convex profile formed between said radiusedends of said crest of said milled teeth.